How to Size your home Solar PV System – Panel and Battery

Updated on September 9, 2017

For a solar PV system to work properly, the size of the panels and the battery must be matched with the energy needs of the appliances. Because panels and batteries are expensive, people often try to save money by installing too few panels or too small a battery. This is very poor practice and does not really save money, because a system that is too small for the appliances do not work well and the battery will have to be replaced, often at high cost.

Sizing PV systems for homes is not difficult if you know what appliances will be used and how long they will operate each day. Because all the power must come from the solar panels, it is most important that they are large enough to provide the energy needed even on cloudy days.

Something needs to be calculated first

1. Calculating appliance watt-hours used each day

To calculate the number of watt-hours needed each day from the panels, first calculate the number of watt-hours needed each day by the appliances. Then increase the result to cover the watt-hours lost in the wiring and battery before the energy reaches the appliances. To calculate appliance watt-hours, multiply the number of watts needed to operate each appliance by the number of hours each appliance is used per day.

Always calculate the energy used on a ‘per day’ basis because the solar panels provide energy on a daily cycle. Sometimes an appliance is used more on some days than on others. For example, the lights in a community center may only be used on Saturdays and Sundays. In that case, calculate the total watt-hours needed per week and divide by 7 to find the watt-hours needed per day.

2. Calculating total watt-hours needed from the panels each day

In a PV system, there are always electricity leaks. Even if the panels produce a full 100 Wh a day, this will not be enough to power appliances needing 100 Wh a day because some of the energy from the panels is lost before it reaches the appliances.

In most solar PV systems, the energy from the panels is first stored in a battery before it is sent to the appliances. Some energy is always lost in the battery. So some of the energy provided by the panels never reaches the appliances. Also, a little energy is always lost in the wires and controller even if the wires are the correct size and the controller is working. For every 100 Wh needed by the appliances, the panels must provide at least 130 Wh. The extra 30 Wh is lost in the battery, wires, and controller.

3. Estimating the energy output from a solar panel

Photovoltaic panels are made in different sizes. The larger the panel, the more energy in watt-hours it will produce. Panel manufacturers rate the size of their panels by the watts of power that they will produce when the sun is at its peak. This is called panel peak watts.

On manufacturers’ data sheets, panel peak watts are usually shown as Wp.

The peak watts produced depend not only on the size of the panel but also on the brightness of the sunlight striking the panel. Therefore, you cannot calculate panel watt-hours by multiplying peak watts by the hours the sun shines, because the brightness of the sun is constantly changing as it moves across the sky and it is only at its peak in the middle of the day.

Even at peak sunlight for the day, the brightness of the sun may be reduced by clouds. So a panel will produce less energy in a cloudy climate than in a sunny climate. To estimate the energy produced by a panel you need to consider both the size of the panel and the climate at the location.

Fortunately, many measurements have been made of the energy output of solar panels that can be used to estimate their energy production. These measurements were made over many years in a tropical Pacific Island location.

They show that it is reasonable to estimate that a 35 Wp solar panel will provide 120 Wh per day on average in a typical tropical, coastal climate such as found in many developing countries. This type of climate rarely has completely clear days, but the clouds are generally scattered and it is unusual to have more than three or four days of continuous cloud.

On very clear days the panel will produce more energy, on cloudier days less, but over many days the average daily output from a 35 Wp panel will be about 120 Wh in this type of climate.

If a larger panel is used there will be a larger output of energy. If the panel is twice the size and rated at 70 Wp, it will produce an average of 240 Wh per day, twice as many watt-hours per day than a 35 Wp panel.

Size your solar panels

1. Calculate the correct of solar panels size

The energy used by appliances is measured in watt-hours and the energy produced by the panels is also measured in watt-hours. Watt-hours of energy are like liters of motor fuel. When 5 liters of fuel are needed to go from one place to another, if only 4 liters of fuel are provided the motor will stop before the trip is completed.

In a PV system, if an appliance needs 100 watt-hours a day to work properly and if the solar panels only produce 80 watt-hours the appliance will stop working early in the day.

Common Mistakes:

The most common reason for the failure of a solar PV system is that the panels are too small. Designs are usually based on new components used under ideal conditions. As all parts of a PV system degrade over time, the system becomes less efficient and the panels must supply more energy as the system ages.

Because panels that are too small do not charge the battery enough each day, battery life will be shorter than in a system with enough panel capacity. Trying to save money by using panels that are too small results in spending much more on battery replacements over the life of the system. It is usually cheaper to add extra PV panels, because battery life is increased and fewer battery replacements will be needed.

2. Finding the number of panels needed

If two panels are joined together, twice as many watt-hours will be produced. Three panels will produce three times the watt-hours, and so on. The watt-hours produced are the same whether the panels are connected in series or in parallel.

To find the number of panels needed in a system follow these steps:

Step 1: Calculate the watt-hours per day for each appliance used.

Step 2: Add the watt-hours needed for each of the appliances to find the total watt-hours per day needed by the appliances.

Step 3: Multiply the total appliance watt-hours per day by 1.3 to find the total watt-hours per day that the panels must provide.

Step 4: Divide the total watt-hours per day by the Panel Generation Factor for your climate (typically 3.43, you can find more details below)

Step 5: Divide the total peak-watt capacity by the peak watts of the panels available to you.

To make a reasonable estimate of the panel output for different climates, you can use the following guidelines to find out number of Panel Generation Factor:

Climate Class 1: Sunnier than the tropical coastal climate with many days of clear skies and few cloudy periods longer than four days. A desert location may be in this class. For this climate, use a Panel Generation Factor of 3.86

Climate Class 2: A tropical coastal climate with most days partly cloudy. Fully cloudy periods are usually no more than five days long. For this climate, use a Panel Generation Factor of 3.43

Climate Class 3: Cloudy periods of five to seven days occur regularly but are typically followed by three or more clear days. For this climate use a Panel Generation Factor of 3.0

Climate Class 4: Cloudy periods of ten or more days occur regularly and fully clear days are unusual. For this climate use a Panel Generation Factor of 2.57

Remember that you must use the cloudiest season for this calculation even though part of the year the climate may be very clear and sunny. If the system is not sized to allow for the cloudy season, then it will not work properly at that time of year.

Size the Solar Battery

A solar battery is needed because the appliances use electricity at different times and at different rates than the panels produce. For the system to work properly, the battery should be of the deep-discharge type and be large enough to store enough energy to operate the appliances at night and on cloudy days.

Also, for the battery to last a long time, it should not be discharged too much or too often. In sizing a battery, it is important to install one large enough to operate the appliances for at least five days without recharging. In climates that have long periods of cloudy weather, a larger battery may be needed.

Remember that battery life depends on how much discharge takes place before a recharge. So another way of sizing a solar battery is that the battery should be large enough so that one day’s use of the appliances will discharge it no more than one-fifth of its full charge. This limited discharge before recharging will help the battery to last a long time.

#Few steps to size Solar Battery

Step 1: Calculate the watt-hours per day used by each appliance.

Step 2: Total the watt-hours per day used by all appliances.

Step 3: Multiply the total appliance watt-hours per day by 5 for a deep-discharge battery, multiply by 7.5 for a maintenance-free battery or multiply by 10 for a vehicle battery.

Step 4: Divide the result of Step 3 by the battery voltage. The result will be the required ampere-hour capacity of a deep-discharge battery at a C10 discharge rate. If the battery you want to use has the ampere-hour capacity rated at the C100 discharge rate, you need to multiply the calculated ampere-hour size by about 1.3. So if you calculate the C10 rate as 100 Ah, you need to buy a battery with a rating of at least 130 Ah at C100.

Panel vs. Battery

The size of the panels and the battery are both determined by the watt-hours used by the appliances. The number of watt-hours changes when appliances are added or removed from the system and when appliances are used more or less each day.

If you have more panels and a larger battery than you need, it is not a problem. It is a problem, however, when the panels or battery are too small. People often want to add appliances to an existing PV system. If they do, the system will not work properly unless the panels and battery are large enough to provide the extra watt-hours. Whenever a new appliance is added, or an old appliance replaced by a new one, it is important to recalculate the correct panel and battery sizes and to increase the system capacity to handle any increased load.

It is also common for people to underestimate the amount of time that lights and other appliances will be used. If the PV system size is calculated using estimates of appliance use that are too low, then the system will not be powerful enough and will not work well.

Increasing the solar panel size increases battery life

It has been shown that increasing the panel size increases battery life, particularly in a climate with frequent cloudy conditions. With the cost of solar panel capacity falling but the cost of batteries slowly increasing, it makes good economic sense to increase the panel size by 20% to 30% over the minimum. This can dramatically improve the reliability of the system during cloudy weather and can greatly extend the life of the battery. This reduces the cost over time as battery replacements are now the most expensive component in a home PV system.

CONCLUSION

Although a smaller system may be cheaper at first, it is often more expensive in the long term. Trying to save money by installing too few panels or too small a battery only leads to an unreliable system with a high maintenance cost.

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